1. Field of the Invention
The present invention relates to a color filter substrate for fringe-field switching mode liquid crystal display and a fringe-field switching mode liquid crystal display in which the color filter substrate is used.
2. Description of the Related Art
Recently, an in-plane switching mode liquid crystal display in which the initial alignment of liquid crystal is horizontal to the surface of a substrate and in which the liquid crystal is rotated horizontally to the substrate surface and a novel fringe-field switching mode liquid crystal display designed to attain an enhanced transmittance have been proposed and are being marketed. These liquid crystal displays are used in a normally black mode (when no driving voltage applied, the liquid crystal horizontally aligned; crossed nicols used as the polarizer), and realize a wide viewing angle and a high contrast, so that they are becoming a mainstream display for use in large-size TVs and mobile devices.
The fringe-field switching (hereinafter referred to as FFS) mode liquid crystal display can realize higher transmittance and higher displayed image quality than those of the conventional in-plane switching mode liquid crystal display. However, the requirement for the electrical properties, especially relative dielectric constant, of color filter and other members, including an insulating layer between the pixel electrode for driving the liquid crystal and the common electrode, for use in the fringe-field switching mode liquid crystal display is becoming severe.
The difference between the in-plane switching mode liquid crystal display and the fringe-field switching liquid crystal display will be described with reference to FIG. 1 and FIG. 2.
FIG. 1 shows a cross section of the in-plane switching mode liquid crystal display. The in-plane switching mode liquid crystal display is so constructed that a color filter substrate 40 and an array substrate 50 are arranged facing each other and stuck together with a liquid crystal layer 46 interposed therebetween. Pixel electrodes 51 and common electrodes 52 are provided on the array substrate 50 with an insulating layer 22 interposed therebetween. The pixel electrodes 51 and the common electrodes 52 are wiring layers comprised of a highly conductive material, such as a metal. They are often provided in a comb-shaped pattern with a pitch of tens of microns. In the liquid crystal layer 46, use is made of a liquid crystal that makes an initial alignment horizontal to the surface of the substrate and exhibits positive dielectric constant anisotropy.
FIG. 1 shows the state of “white display” in which, for example, a liquid-crystal-driving voltage of 5 V is applied between the pixel electrodes 51 and the common electrodes 52. Electric field is applied in a lateral direction as indicated by a line of electric force 43 in FIG. 1, so that the liquid crystal molecules between the pixel electrodes 51 and the common electrodes 52 are horizontally rotated by the applied voltage. The liquid crystal molecules 47 close to the substrate surface cannot attain satisfactory rotation because of a strong restraining force of rubbing on the alignment film. In FIG. 1, not only the liquid crystal molecules 48 on the pixel electrodes 51 but also the liquid crystal molecules 49 on the common electrodes 52 remain in the initial horizontal alignment and are not rotated because the application of the voltage for rotating the liquid crystal molecules is poor (liquid crystal molecules 49 are oriented in the direction perpendicular to the sheet). This means that even when, for example, the pixel electrodes 51 and the common electrodes 52 are formed of a transparent conductive film, such as ITO, liquid crystal molecules that do not rotate regardless of the application of a driving voltage are left, causing a lowering of transmittance.
FIG. 2 shows a cross section of the fringe-field switching liquid crystal display. The fringe-field switching liquid crystal display is so constructed that a color filter substrate 40 and an array substrate 60 are arranged facing each other and stuck together with a liquid crystal layer 56 interposed therebetween. Pixel electrodes 61 and a common electrode 62 are provided on the array substrate 60 with an insulating layer 22 interposed therebetween. Both the pixel electrodes 61 and the common electrode 62 are formed of a transparent conductive film, such as ITO. A characteristic feature of this electrode structure is that within a pixel, the common electrode 62 is provided in a solid planar form, while the pixel electrodes 61 are provided very finely with an electrode width (WL) of about 2 to 10 μm and with a pitch of 15 μm or less.
For example, the pixel electrodes 61 can be provided with an electrode width (WL) of 5 μm and a pitch of 11 μm. The smaller the electrode width (WL) and pitch of the pixel electrodes 61 are, the greater the contribution to the increase of the transmittance of the liquid crystal display is. The reason therefor is that the fringe electric field provided from the pattern edges of the pixel electrodes 61 to the common electrode 62 is the agent of liquid crystal drive. When the electrode width (WL) is, for example, 2 μm, rendering the inter-electrode distance (Ws) a little bit large, about 3 μm, realizes high efficiency from the viewpoint of transmittance.
In the liquid crystal layer 56 shown in FIG. 2, liquid crystal molecules are rotated across substantially all the area within the pixel by the application of a liquid crystal driving voltage between the pixel electrodes 61 and the common electrode 62, thereby realizing a display of high transmittance. In a liquid crystal display device in which a liquid crystal of initial horizontal alignment is used, the FFS mode can be regarded as means for transmittance enhancement by the generation of fringe electric field at a short cycle.
Known technologies relating to the color filter for in-plane switching mode liquid crystal display are disclosed in Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 2009-229826 and JP-A-H9-292514. The technology disclosed in JP-A-2009-229826 comprises specifying the dielectric dissipation factor and chromaticity of a color filter applicable to an in-plane switching mode liquid crystal display. The relative dielectric constant required for the in-plane switching mode is claimed therein. However, there is no disclosure of particular values with respect to the relative dielectric constant of each of red, green and blue color layers, and there is no disclosure with respect to a requisite average of relative dielectric constant and an extent of variation thereof.
Therefore, naturally, no attention is drawn to the respective dielectric constants to be uniformly exhibited by red, green and blue color layers, which are required at the time of halftone display. Moreover, there is no disclosure at all with respect to the influence of the dielectric constant of a black matrix usually provided for ensuring contrast at the time of color display by such color pixels. There is no description with respect to the relative dielectric constant of a color filter needed in the fringe-field switching mode liquid crystal display that while realizing a high-transmittance display, requires high-level electrical properties on liquid-crystal-surrounding members. Further, no study is made with respect to the liquid crystal driving frequency (120 Hz, 240 Hz) prevailing in liquid crystal televisions in which the in-plane switching mode or fringe-field switching mode is employed. The relative dielectric constant at the frequency is not disclosed. The electrical properties of liquid-crystal-surrounding members often change the values thereof in low-frequency regions and high-frequency regions, so that they should be measured in actual use conditions.
In the structuring of the color filter described in JP-A-2009-229826, an overcoat layer comprised of a transparent resin layer is avoided. In the FFS mode liquid crystal display device, an overcoat layer comprised of a transparent resin layer must be provided on the color layer of the color filter thereof. The reason therefor is that there are extremely high-level requirements with respect to the voltage holding ratios of liquid crystal materials for use in the FFS mode, and that in order to avoid any lowering of voltage holding ratio attributed to any adverse effect of impurity ions contained in an organic pigment of the color layer, it is essential to provide an overcoat layer comprised of a transparent resin layer.
JP-A-H9-292514 discloses a color film whose relative dielectric constant is 4.5 or below, and in paragraph 0014 discloses a pigment black 7 comprised of carbon as a black pigment of black matrix. However, with respect to the color films disclosed in FIG. 3 of JP-A-H9-292514, for example, the relative dielectric constant at a frequency of 100 Hz of blue color film can be read as about 3.9, that of red color film as about 3.45 and that of green color film as about 3.1. The variation of relative dielectric constant is large, so that the color films can be applied to the conventional in-plane switching mode liquid crystal display but cannot be applied to the display of high image quality by fringe-field switching mode liquid crystal display. JP-A-H9-292514 does not disclose any concept of uniformizing the relative dielectric constants of individual color films. Further, there is no disclosure with respect to any impact of the relative dielectric constant of black matrix on the FFS mode liquid crystal display device.